The present invention relates to a switching current amplifier for inductive loads and more specifically for inductive loads such as stepping motor windings. It relates to Applicant's patents, U.S. Pat. No. 4,100,471 and U.S. Pat. No. 4,140,956, a divisional of U.S. Pat. No. 4,087,732. These patents disclose switching current amplifiers for bifilar wound stepping motor inductances.
Switching amplifiers are ideal for stepping motor applications in that they provide current limiting, low dissipation, fast current rise and fast response. There are generally two approaches using switching amplifiers; the single high voltage pulse per step with low voltage hold or the continous chopping from a high voltage. This invention is concerned with the latter method. The use of a switching amplifier necessitates the mixing of a high frequency chopping signal and a position command signal to produce effective currents in the motor windings to provide the desired motor response. One of the methods to provide the chopping in the use of a fixed frequency clock to turn on the high voltage to the motor. The motor current is allowed to rise until it reaches a particular level whereupon it is turned off until the next clock pulse. Another method uses a capacitive delay circuit to turn off the motor current for a fixed period whenever it exceeds the required amount. In both of these methods the motor current is subject to the arbitrary fixed timing as well as to the variations dependent upon the high voltage supply level, the motor inductance and the motor back emf. An example of the deleterious effect of an arbitrary timing circuit can be seen in FIG. 1C. When the desired motor winding effective current represented by G is stepped down, the motor current would be allowed to decrease only to point H where an arbitrary timing pulse would turn on the output devices thus disrupting the rapid decrease required for precise motor control. Ideally, precise motor control is represented by the solid curve I in FIG. 1C wherein the current is allowed to drop unperturbed.
The output circuits provided an Applicant's U.S. Pat. Nos. 4,100,471 and 4,140,956 provides a means to test for the low current level as well as the high current level and to switch the amplifiers at the proper time, for example, to allow the current to drop to point K before the current is switched on again. Point K in FIG. 1C represents a level demanded by the position command signal. The parameters of the amplifier disclosed in U.S. Pat. No. 4,140,956 and shown in FIG. 2 of this specification are selected to provide high and low switching at an optimal frequency, low enough to reduce the dissipation in the output transistors but high enough so that the motor will not respond thereto. It is also desirable to select the frequency above the audio range so that the switching is not audible.
However, the prior art switching amplifier shown in FIG. 2 suffers from two disadvantages that are caused by the leakage inductance in the windings and associated wiring. First the leakage flux causes an increase in power dissipation in the output transistors. Second, the flyback requires inductive flyback clamping means (shown in dashed lines) and current flowing in the clamping means is not sensed in the sensing resistors 174, 176 which give spurious inputs to the comparator 182 causing the amplifier to switch at a too high frequency. This high frequency also causes an increase in power dissipation in the output transistors. Each of these factors, i.e., increased power dissipation caused by the leakage flux and the increased power dissipation caused by the too high frequency switching, can cause the output transistors 160 and 162 to fail besides providing imprecise motor control.
A bridge configuration such as in FIG. 3 might be used in an attempt to avoid the leakage inductance problem. However, the bridge configurations can still either produce sizeable transient error currents that are sensed by the sense resistors or have motor currents that bypass the sense resistors and, therefore, are not measured. Another method that could ideally be used is to place sensing devices in series with the motor windings to measure the currents directly. However, at the high switching frequencies involved, the prior art devices suffer from the inability to avoid the error caused by the common mode signal accompanying the signal produced by a current sensor.
Ideally, for precise, accurate switching of the chopping amplifier a device is required that can measure the load current directly, i.e., be placed in series with the load winding, that has a large high common mode signal rejection, that can determine the high and low current requirements in reference to the position command signal, and that can control the switching at the proper time.